Ultrasound analysis has only very restricted use for uncrosslinked rubber mixtures comprising fillers or comprising other substances, because of the high level of attenuation of the sound. A simple and rapid method is required for the determination of particulate contaminants and the degree of dispersion thereof in an uncrosslinked rubber mixture.
Rubber is used for various technical components, such as tires, engine bearings, O-rings, etc. Even individual coarse-particle contaminants≧100 μm lead to early failure of the rubber components, in particular under dynamic load [A. Schrdder et al., Kautschuk, Gummi Kunststoffe, 11, pp. 584 to 596, 2008]. In the case of very thin-walled components (1 mm), such as O-rings, this type of particulate contamination can impair sealing properties. In the case of profiles, the appearance of the components requires that particulate contaminants be avoided.
The degree of dispersion of the substances within the rubber, in contrast, also determines the dynamic-mechanical property profile of the crosslinked rubber mixture and thus determines the quality of the technical components composed thereof. By way of example, hysteresis increases, and with this the rolling resistance of a tire increases, if the reinforcing nanoscale fillers are inadequately deagglomerated and dispersed [Payne, A. R.; Watson, W. F.: Rubber Chem. and Technol. 36 (1963) No. 1, pp. 147-155].
Until now, the quality of an uncrosslinked rubber mixture has been monitored only by taking individual samples (<0.1% of the total quantity). The Mooney viscosity of the rubber and the crosslinking curves of the individual samples are determined. The tensile properties of the mixture are determined after vulcanization of individual samples [J. Schnetger, Lexikon Kautschuktechnik [Encyclopedia of rubber technology], Heidelberg, 3rd revised edition (2004), 388-391]. These tests do not reveal small numbers of individual coarse-particle contaminants. On the basis of the test results it is moreover impossible to decide whether any deviation from the specifications involves particulate contaminants or other defects. Individual samples are also subjected to optical microscopy [J. Schnetger, Lexikon Kautschuktechnik [Encyclopedia of rubber technology], Heidelberg, 3rd revised addition (2004), 448-449]. Although this can reveal individual contaminants, full quality control of the rubber mixture is not achieved. Furthermore, it is advantageous to vulcanize the sample. The individual samples are not representative, because of their small number. The assessment of the samples by what is known as the Cabot method for deagglomeration of carbon black materials is moreover subjective. The quality controls described are moreover carried out manually, and not before some minutes or indeed days have expired since production of the rubber mixture. The method is time-consuming, since the test is carried out manually and off-line.
Ultrasound is used for the determination of coarse defects in materials which are good ultrasound conductors, for example metals or liquids. Measurements here are made in reflection by the pulse-echo method. The ultrasound frequency spectrum can also be used to determine the particle size distribution in low-viscosity suspensions, for example aqueous suspensions or polymer melts [U.S. Pat. No. 5,121,629]. The rubber industry also uses ultrasound for the quality control of crosslinked components [J. Schnetger. Lexikon Kautschuktechnik [Encyclopedia of rubber technology], Heidelberg, 3rd revised addition (2004), 557-558].
In contrast, unvulcanized rubber mixtures are very poor conductors of ultrasound, because of their viscoelastic properties, and because the content of nanoscale fillers is often very high [J. Schnetger, Lexikon Kautschuktechnik [Encyclopedia of rubber technology], Heidelberg, 3rd revised addition (2004), 557-558]. The ultrasound therefore penetrates only to a small depth into an uncrosslinked rubber mixture. It is therefore possible to test thin-walled unvulcanized rubber samples which the sound is just capable of penetrating. In J. Kirchhoff, et al., Gummi Kunststoffe, 55 (2002) 373-381, the crosslinking behavior of rubber mixtures is carried out by taking individual samples measuring 2 mm. In EP-2314442 A, the quality of a rubber mixture in respect of the dispersion of crosslinking chemicals is determined by means of a marker with particle size<100 μm and with density>2 g/cm3. The thickness of the sample here is only 4 mm.
There is therefore no process known within the prior art that permits very simple, unambiguous, rapid and effective quality control of uncrosslinked rubber mixtures.
It was therefore an object of the present invention to provide a novel and easily operatable process for the rapid and dependable control of the quality of uncrosslinked rubber mixtures, i.e. a process suitable for the detection of particulate contaminants with diameter>10 μm, and also an apparatus suitable for said use.